With the development of display technology, a new generation of organic light emitting diode (OLED) display devices have advantages such as a lower manufacture cost, a higher reaction speed, a higher contrast, a wider visual angle, a larger range of working temperature, requiring no backlight unit, being bright-colored and having light weight as compared with conventional liquid crystal display (LCD) devices. Hence, the OLED display technology has become the fastest growing display technology at present.
A main development direction of the current OLEDs is to control the current between the source and the drain of the driving transistor by changing the gate voltage of the driving transistor that drives the OLED to emit light directly, so as to realize variation of the light emitting brightness. In this way, the driving transistor will be in the working state for a long time, i.e., the gate thereof keeps in a positive voltage or negative voltage state all the time. The existing thin film transistors (TFTs) of different materials generally have characteristic such as the threshold voltage Vth and the mobility thereof vary with the passage of time under DC bias (the varying trends of the positive voltage and the negative voltage are opposite). Therefore, after the OLED display panel works for a period of time, phenomena such as variation in gray scale brightness and non-uniform gray scale (caused by different variations of the characteristics of different TFTs) may come up easily.